1. Field of the Invention
The present invention relates generally to aluminum alloys as well as to materials and methods that include aluminum alloys. It is most particularly adapted to aluminum brazing sheet materials suitable for use as evaporator plates, but it is also adaptable to other aluminum alloys.
2. Description of Related Art
The precipitation characteristics of aluminum alloys must be considered frequently during evaluation of materials for particular uses. This is because the presence or absence of such precipitates in aluminum materials greatly affects the ability of the alloy to withstand exposure to extreme temperatures, chemicals and/or impact over the long and short term. Evaluating the presence (or absence) of particular precipitates is also necessary for determining standard practices for downstream manufacturing operations (such as hot forming and straightening, adhesive bonding, painting and dry-film lubricant curing). The state of precipitation that exists in an alloy at the time of reheating plays a significant role in the effects of reheating or annealing.
Annealing treatments employed for aluminum alloys are generally selected based on the alloy type as well as on the initial structure and temper. Depending on the final properties desired, it is possible to subject a material to either a full anneal, a partial anneal, or what is known as a stress-relief anneal. The softest, most ductile and most workable condition of both non-heat-treatable and heat-treatable wrought alloys is produced by a full anneal to the temper designated xe2x80x9cO.xe2x80x9d Aluminum sheet products that have been cold worked and then annealed to the O temper are generally recrystallized.
When articles comprising aluminum alloy sheets with a core and a cladding of aluminum brazing alloy are being formed, the assembly of shaped parts is brazed by subjecting the assembly to a temperature above the melting point of the cladding and below the melting point of the core metal. Any substantial coarsening of the metal grains that is brought about at the elevated temperature necessary to effect the brazing, tends to encourage penetration of the molten brazing alloy through the core and cause weakening and ultimate failure of the assembly. This problem is disclosed in U.S. Pat. No. 3,966,506, [xe2x80x9cthe ""506 patentxe2x80x9d] the content of which is incorporated herein by reference. Brazing sheet materials are generally comprised of multiple layers. See, e.g. U.S. Pat. No. 5,292,595 that discloses a 3-layered clad material. In addition, materials such as A3005 and A3003 have been used as the core material and a clad layer of another aluminum alloy such as A7072 or A4343. Typically, the aluminum alloy brazing stock material comprises a core that has been clad on one or both sides with an aluminum based alloy. The composition of the core and the clad are carefully selected and are important in the properties of the resulting brazing sheet material.
Substantial problems exist with corrosion from both the interior and exterior of the tube. Particularly, problems arise in the interior of the tube due to erosion/corrosion which develops due to the extremely fast velocity of coolant that is moving through the interior of the tubes. The coolant traveling at high pressures and velocities will cause erosion. (The phenomenon of internal corrosion/erosion is not to be confused with core erosion or liquid film migration (LFM), which occurs during the brazing process. For clarification, internal corrosion/erosion describes the corrosion/erosion of the interior of the article. Internal corrosion/erosion occurs when the inner surfaces are exposed to the coolant traveling at high velocity. External corrosion describes the corrosion of the exterior of the article. External corrosion occurs when the external surfaces are exposed to abrasive/corrosive materials such as road salt, humidity in the air etc. Core erosion or LFM, which occurs during the brazing process, is the phenomenon whereby the clad material penetrates the core material resulting in xe2x80x9cerosionxe2x80x9d of the core.)
One problem with recrystallized materials lies with their susceptibility to core erosion during brazing. While recrystallization is desirable in order to make the material softer and more formable, core erosion resistance of such recrystallized materials is typically insufficient. Poor core erosion resistance is not acceptable for brazed materials since corrosion/erosion resistance is extremely important for the end use products that will be formed. Core erosion during brazing will result in a material much less resistant to corrosion/erosion and a material with much less strength. While the ""506 patent describes methods for forming materials that are said to retain some unrecrystallized grain fragments dispersed among the recrystallized grains in a metal sheet in order to prevent or minimize grain coarsening when the shaped sheet is heated to an elevated temperature, no substantial benefit in terms of core erosion resistance of such materials is taught. Moreover, the core material of the ""506 patent will also lack the formability properties of recrystallized materials.
As such, the provision of a material that is useful as core material in brazed sheets (such as those used to manufacture evaporator plate) as well as other applications where both satisfactory formability and core erosion resistance during brazing are necessary would be highly desirable. It would also be desirable in the industry to obtain a product which is not susceptible to core erosion or LFM during brazing, and has the additional benefit of greater strength, better fatigue resistance, and improved erosion/corrosion resistance during operational use of the product. In particular, a true long-life evaporator alloy has never before been made, and such an alloy would be capable of meeting this very important market need.
In accordance with these and other objects, the present invention provides aluminum alloys and layers formed in aluminum alloys as well as methods for their manufacture. Aluminum alloys of the present invention are provided with at least one discrete and a substantially continuous layer of uncrystallized grains abutting the surface of the core sheet, which can be clad or not.
Materials of the present invention can be formed, for example, by a process that includes a final anneal that permits softening of the material. Processes of the present invention substantially recrystallize the material but leave a discrete and substantially continuous layer abutting the surface roughly up to 20%, preferentially less than 5% by volume of the material unrecrystallized. In preferred embodiments, the aluminum material is a core material that is clad on one or both sides and the discrete uncrystallized layer forms at the boundary between the clad and the core.
Additional objects, features and advantages of the invention will be set forth in the description which follows, and in part, will be obvious from the description, or may be learned by practice of the invention. The objects, features and advantages of the invention may be realized and obtained by means of the instrumentalities and combination particularly pointed out in the appended claims.